Understanding MPLS: MPLS Architecture May 14
To further understand exactly how MPLS works you must understand how the control plane and data plane compliment each other during the forwarding process in a Label Switch Router. This process is slightly different depending if you are on a PE or P device. Below is a graphic to help illustrate those slight differences.
As you can see from the graph the Label Switch Routers exchange routes with each other, usually by the routing protocols OSPF or EIGRP. This is a standard network layer function. The best routes for the respective networks based on the routing protocols in use are placed in the routing table, this is how standard routing works.
Once the routing table is populated, CEF (Cisco Express Forwarding) uses that information to enable MPLS label switching. Simply put, CEF is required to be able to label switch in an MPLS network. CEF has two components, the Forwarding Information Base (FIB) and the adjacency table. The FIB (located in the data plane) is responsible for maintaining next hop IP addresses for all of the routes in the routing table. The adjacency table is responsible for maintaining the layer 2 information for each FIB entry. The adjacency table is responsible for the layer 2 rewrite, and it avoids the need for an ARP request for each IP address lookup. Basically CEF binds the next hop address for a specific network to a physical interface mac address. It relies on recursive updates in and from the routing table to do this. This is essentially what allows layer 3 switching.
When you enable MPLS on a router, the routing table is also copied in to a MPLS IP routing control table, which remains in the control plane. Adjacent to this is the Label Information Base (LIB, also referred to as Tag Information Base as shown in the picture) which is where the MPLS labels exist. The MPLS IP routing control table is what actually binds labels from the LIB to the IP routes in the IP routing table. The MPLS IP routing and control table is also where the label distribution protocol lives. Said protocol, like Label Distribution Protocol (LDP), shares the locally significant label to IP route bindings with other LSR’s in the network. This makes the creation of virtual circuits via label stacks possible. Label stacks are used in MPLS applications such as Traffic Engineering and VPN implementation.
MPLS IP routing and control information is also copied in to the FIB and the TFIB/LFIB (Tag or Label Forwarding Information Base). The difference between these two tables (FIB and TFIB/LFIB) is in their purpose in the forwarding of data. What is not shown is a logic block that exists in between the FIB and TFIB/LFIB. This block is where label lookup occurs and the decision to remove the label from the packet for forwarding, or replace the label with the locally significant label for forwarding occurs. This is applicable when a labeled packet is received. Since the packet is labeled it goes to the TFIB, there is usually an arrow pointing up to the FIB (denoting the logic process) but not in this particular diagram. If the label was removed it would be sent to the FIB for appropriate forwarding.
The MPLS Edge router has the most intricate architecture because it must be able to forward data on to and off of the MPLS network, to and from the customer. An understanding of basic MPLS architecture is paramount to understanding the configuration of MPLS devices in your network.